Silicon is a quite brittle material which is quite difficult to slice thinner than approximately 250 microns. Since the minority electrical carrier diffusion length through single-crystal silicon material averages about 200 microns at an intensity of one sun through air mass one (AMl), only a relatively small proportion of the electrical carriers generated by incident solar radiation can be collected in a silicon cell having such a thickness as 250 microns. Therefore, the overall conversion efficiency of such thick silicon solar cells is low.
Consequently, attempts have been made in the prior art to produce thinner, wafer-type silicon solar cells. Prior art attempts to produce silicon photovoltaic cells much thinner than 250 microns have involved expensive, time-consuming lapping procedures for producing a micro-smooth surface, followed by expensive, time-consuming etching procedures considerably raising the cost of the completed cell and considerably limiting the overall production rate capability. Even when these procedures are used to produce thin silicon solar cells, their conversion efficiencies in commercially produced wafer-type units seldom exceed 15% at one sun at AMl.
Attempts have been made in the prior art to pull ribbons of silicon directly from a melt. To date such ribbon-pulling-produced solar cells have been plagued by defects and have occasionally, but not consistently, exhibited conversion efficiencies in laboratory-produced (not commerically produced) units up to approximately 16% at one sun at AMl, with a ribbon thickness of aproximately 100 microns. The rate at which each such ribbon has been pulled from a melt is very slow, being of the order of one to twenty square centimeters of ribbon per minute at a thickness of less than 150 microns.
As used herein, "one sun" means the intensity of solar radiation as actually received from the sun without augmentation.
As used herein, "air mass one" or "AMl" means the average maximum solar radiation received at the earth's surface at sea level resulting from solar radiation passing vertically through one thickness of atmosphere.